Please open a SR!
I assume that you started with "old" Messenger linked to eDirectory. The migration path (from eDir to GroupWise) did not really take care of the new situation - that's the dilemma you have to deal now. In my eyes it's a bug ...
I have a customer running into the same situation. He did not get a solution, but explanation.
Promised update. This is what I got back from development.
Migrated users are in a special class all their own. If you want to dig into it you can pull up the Arango web interface and look at the details of the user object. There are 4 types. 0 = migrated 3.x user. 1 = eDir associated user. 2 = GroupWise sourced user. And 3 = AD sourced user.
You can disassociate a type 0 and then re-associate as a type 1 or even associate with a GW account. But beware, there is no way to go back to a type 0 unless you can re-migrate from the original source.